Field
This disclosure relates to a binder for a rechargeable battery, a separator for a rechargeable battery, and a rechargeable battery including the same.
Description of the Related Technology
Recently, as various electronic devices have reduced in size and weight, there is a need for rechargeable batteries as a power source for these electronic devices that have high-capacity, and are smaller in size and weight as well.
In particular, research for developing a rechargeable lithium ion battery has been high and simultaneously, manufactured and sold due to benefits such as a high voltage, a long cycle-life, high energy density, and the like.
These characteristics of the rechargeable lithium ion battery may be largely affected by characteristics of an electrode, an electrolyte solution, other battery materials, and the like.
In particular, a separator for the rechargeable lithium ion battery influences the cycle characteristics of a rechargeable lithium ion battery. The separator may be manufactured by coating a slurry including an inorganic particle and a binder on a porous substrate and is called as a coating separator because it includes a coating layer including the inorganic particle and the binder formed on the porous substrate.
The binder plays a role of bonding the inorganic particle with a porous polyethylene film used as the porous substrate.
When this separator is used to manufacture a rechargeable lithium ion battery, cycle characteristics of the rechargeable lithium ion battery may be determined by characteristics of the binder bonding porous polyethylene (PE) with ceramic particles (a kind of inorganic particle), and also by characteristics of the ceramic particles themselves.
Particularly, when ceramic particles having high heat resistance are used to form the coating layer, a battery having a high voltage and high-capacity characteristics may be manufactured. However, the ceramic particles having high heat-resistance may sufficiently exert their own characteristics, when the binder stably maintains the structure of the ceramic coating layer during charge and discharge of the rechargeable lithium ion battery. In other words, the high heat-resistance ceramic particles need to maintain their stability inside the separator.
In addition, the binder needs to firmly bond the separator with each electrode.
Therefore, a binder having excellent heat resistance and adherence is required.
Specifically, a binder composition (hereinafter, referred to as PVDF-based binder) obtained by mixing a polyvinylidene fluoride (PVDF)-based polymer as a representative binder for an electrode with an organic solvent such as N-methyl-2-pyrrolidone (NMP) and the like tends to be used as the binder for the coating separator. However, the PVDF-based binder needs to be used in an excessive amount for a separator to maintain sufficient adherence and is difficult to continuously maintain the stable structure of a coating layer by impregnating an electrolyte solution and moving lithium ions during charge and discharge of a rechargeable lithium ion battery.
In order to solve this problem, a method of improving adherence to an inorganic oxide by using porous polyethylene, a binder having a strongly adhering chemical structure, and a silane coupling agent, a method of using a binder including an IPN (interpenetrating polymer network)-type resin obtained by polymerizing a PVDF-based polymer and a hydrophilic polymer (unsaturated carboxylic acid) (for example, Japanese Patent Laid-Open Publication No. 2013-211273), or the like has been suggested.
However, the binder satisfies the adherence requirement to either the porous polyethylene or the inorganic particle but does not have sufficient adherence to both of them.
Accordingly, when a rechargeable lithium battery using a coating separator formed by using the aforementioned binder is repeatedly charged and discharged, the coating layer is structurally changed and as a result battery capacity deteriorates.